1. Field of the Invention
The present invention generally relates to integrated circuit devices and, more particularly, to an integrated circuit device acting as a transmission power amplifier circuit for a code division multiple access (CDMA) modulation portable terminal, in which a setting bias of an amplifier is switched at of low power output and high power output and a bipolar transistor is employed for raising efficiency at low power output.
2. Description of the Prior Art
In a transmission power amplifier circuit for a CDMA modulation portable terminal, since power control is performed during communication, high priority is given to not only additional efficiency at the time of high power output but efficiency at the time of low power output. Thus, the amplifier is operated by switching a setting bias of an amplifier at the time of low power output and high power output. Therefore, a conventional power amplifier circuit disclosed in, Japanese Patent Laid-Open Publication No. 6-13816 (1994) includes a high-frequency transistor for amplifying a signal and a bias circuit for supplying a base bias to the high-frequency transistor such that a bias switch for changing a setting bias of the high-frequency transistor is provided in the bias circuit. Meanwhile, another known power amplifier circuit disclosed in, Japanese Patent Laid-Open Publication No. 2001-274636 (2001) includes a bias circuit in which a bias generating circuit and a reference voltage generating circuit acting as a temperature compensation circuit for temperature compensation of the bias generating circuit are provided.
In a prior art power amplifier circuit in which the configurations of the above conventional power amplifier circuit and the above known power amplifier circuit are combined with each other, the bias changeover switch is turned off at the time of high power output of the high-frequency transistor such that a voltage from the bias circuit is supplied to the high-frequency transistor as it is. The base bias of the high-frequency transistor at this time is set such that the high-frequency transistor fully satisfies distortion characteristics at the time of high power output.
On the other hand, at the time of low power output of the high-frequency transistor, the bias changeover switch is turned on, so that electric current flows through the bias changeover switch such that the base bias of the high-frequency transistor is lowered. At the time of low power output of the high-frequency transistor, since the high-frequency transistor lies in a linear operation area even if the base bias is lowered, the high-frequency transistor can operate under conditions in which the high-frequency transistor is capable of fully satisfying the distortion characteristics.
By this base bias changeover function of the bias changeover switch, the high-frequency transistor can raise efficiency at the time of low power output having the linear operation area. Meanwhile, in order to greatly raise efficiency at the time of low power output of the high-frequency transistor, it is effective to change over a collector supply voltage of the high-frequency transistor such that at the time of low power output, the collector supply voltage is lowered to a level at which the distortion characteristics of the high-frequency transistor are satisfied.
In the above described prior art power amplifier circuit including the bias changeover switch for changing over the base bias of the high-frequency transistor, the bias changeover switch changes over the base bias set by the bias circuit so as to reduce idle current of the high-frequency transistor such that efficiency at the time of low power output is raised. The efficiency at the time of low power output exhibits an increase of 1 to 2%.
Meanwhile, in case the collector supply voltage of the high-frequency transistor at the time of low power output is lowered, for example, from 3.5 V to 1.2 V, an increase of the efficiency amounts to 15% approximately.
If the function of changing over the base bias of the high-frequency transistor and the function of changing over the collector supply voltage of the high-frequency transistor are performed in combination, the efficiency is raised largely. However, in order to impart these changeover functions to the prior art power amplifier circuit, such problems arise that the number of components of the portable terminal increases and the portable terminal becomes larger in size resultantly.